1. Field of the Invention
The invention concerns a method for making a diffraction lattice on a semiconducting material.
2. Description of the Prior Art
There are known ways to make semiconducting lasers having diffraction lattices. These are Distributed Feed Back (DFB) lasers or Distributed Brag Reflector (DBR) lasers. These lasers are very valuable because of their purity and spectral stability, especially for applications in long-distance telecommunications and in coherent links. Moreover, the structure of these lasers makes them particularly suited to integration, as the presence of the diffraction lattice makes it possible to get rid of at least one of the cleaved faces forming the Perrot-Fabry cavity in a standard laser. These structures can therefore be used to simplify the optical connections with other opto-electronic elements such as modulators, photodiodes, multiplexers, etc.
However, prior art methods for making a diffraction lattice on a semiconducting material are difficult to implement, and it is very difficult to control the characteristics of the lattice. The making of a diffraction lattice is the most difficult step in the making of lasers of this type. Moreover, it is desirable to improve the optical efficiency of this type of lattice.
For example, a DFB laser has at least:
one optical confinement layer, epitaxially grown on a substrate;
an active layer;
a layer, called a guide layer, on which a diffraction lattice is etched.
The standard way to etch a diffraction lattice is to deposit a layer of photosensitive resin on the guide layer, then cut out a mask in the resin, either by UV holography or by electronic masking, and then subject the guide layer to attack by a chemical agent which removes a part of its thickness in the places which are not shielded by the resin mask. It is shown that, to have high efficiency in the manufacture of DFB lasers, a well-determined etching depth of about 0.04 microns should be obtained. The width of the raised parts should be between 0.05 and 0.15 microns. The lattice pitch is about 0.47 microns: this means that the width of the resin strips forming the mask should be at least about 0.15 microns. It is very difficult, with the standard chemical attacking technique, to obtain precise and homogeneous etching depth throughout the surface of the lattice. An object of the invention is a simple method, with high manufacturing efficiency, to make diffraction lattices of this type on a semiconducting material.